#include<iostream>
using namespace std;
string s,t;int d,m,c[99],f=1,p='x';
main()
{
	cin>>d>>s>>t;s+=t;
	for(int i=0;i<14;i++)p!=s[i]?c[++f]++,p=s[i]:++c[f];
	p=='x'&&(c[f]=0);c[1]=0;
	for(int i=1;i<99;i+=2,m<p&&(m=p))p=c[i]>d?c[c[i-1]<c[i+1]?i+1:i-1]+d:c[i-1]+(c[i]?:d)+c[i+1];
	cout<<m<<endl;
}